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Ovulation typically occurs two weeks after the beginning of menstruation, and this is when conception is most likely to occur. For this reason, if expecting mothers cannot pinpoint the exact conception date, the due date is usually set at 40 weeks after the last menstruation.

Doctors prefer to confirm the due date with an ultrasound and adjust as necessary. During an ultrasound procedure, doctors measure the fetus. The measurements tell doctors how far along in gestation the fetus is, which is the gestational age. While doctors can perform ultrasounds as early as five weeks after the last menstruation, the most accurate results occur between eight and 18 weeks. Doctors use the gestational age along with the calculated day of the first day of the last menstrual period to confirm a solid due date.

Calculating a firm due date is not exact, though. Women's periods may not be regular, so the conclusion of gestation may not occur exactly 40 weeks after the first day of the last period. Likewise, fetuses are sometimes abnormally large or small, making confirmation via ultrasound of the exact due date impossible.